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Technical Design Question, Wooden Walls Still Need Tension Band?

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Old 07-17-2014, 10:51 AM   #1
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Default Technical Design Question, Wooden walls still need Tension band?

Hi all,

I'm trying to design a yurt-like structure, in the sense that it is roundish in shape, and was hoping someone might have some engineering expertise on the matter. I'm considering making it approx 12' diameter on the interior, with solid 2x4 walls that can be insulated. I'm considering making it 12-sided rather than a true circle, to make it easier to install standard doors and windows mainly, and I've also considered making the walls tapered, ala Bill Copperthwaite's design.

My question is:

TENSION BAND
If I made the walls plumb, and they were framed with 4x4 posts, and they had a double top plate that overlapped the seams, would that be strong enough to resist the outward roof thrust or would I still need a tension band? I've thought of either cutting the top plate out of plywood or 2x12 or whatever is required to get the radius, and possibly laminating the two layers together for extra strength? And possibly some kind of metal bracket or fastener to connect the posts to the top plate. The roof is obviously pretty small and light at 12' diameter, and it will be built up from the inside out: 2xrafters, t&G decking, rigid foam

insulation

, metal roofing. Basically I'm wondering if the strength of the solid wood framed walls, locked together with the top plate as well as horizontal blocking in the walls (which will serve as nailers for the vertical wood siding) is enough to counter the roof thrust? Or if I should just throw a cable in there for extra security?

If I tapered the walls, I suspect I would need the cable regardless. Has anyone out there built a taperd wall yurt like Bill Copperthwaite has designed and promoted and have any feedback on it?


Thanks!

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Old 07-17-2014, 03:56 PM   #2
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Default Re: Technical Design Question, Wooden walls still need Tension band?

I am unfamiliar with the Bill Copperwaite design but I can comment on what you have said in the post.

First off, the cable is there in a normal yurt because the rafters are not tied together and they would just spread apart if not secured. Since it sounds like your roof, rafters and all, will be tied together with plywood and metal, oppose to fabric which doesn't tie anything in, the whole roof acts as a single unit. So the force at the top of your 2x4 wall would be vertical, not angular. I am not sure if you plan on connecting the ends of your rafters at the wall side but I would do this as well.

Now I don't know if you get snow but snow could possibly press your roof out, maybe not much because of its size and the plywood/metal roof, if you might get a snow load I would consider anchoring the bottom of your 4x4s maybe dig them down a few feet add a little bit of concrete as well, then I don't think a hurricane would budge your yurt.

How do you plan on connecting the 2x4 walls? If you screw them all together that will act like a

tension cable

as well.
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Old 07-17-2014, 05:00 PM   #3
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Default Re: Technical Design Question, Wooden walls still need Tension band?

Thanks for the reply,

I was not planning on using plywood to sheath the roof, rather 1x6 t&g cedar as the exposed ceiling, covered by

insulation

and

moisture

barrier and metal roofing. So the roof assembly would not be quite as rigid as if I used plywood panels, but rigid enough to prevent racking I'm pretty sure. I am definitely planning to block between the rafters at the plate and also lock in the ends with solid 2x fascia.

Quote:
So the force at the top of your 2x4 wall would be vertical, not angular.
Is this really the case? If you have your forces, as in weight of the roof, coming to the wall at an angle, doesn't there have to be some kind of horizontal component resisting the thrust? This is achieved by the tension band in yurts and most commonly in my experience with rafter ties on a gable roof for instance. This is the part I'm confused about, because I know that the top plate could be fashioned as one continuous piece of wood, but don't know how strong that would be. Also, unless I used brackets, hurricane clips or the like, the only connection resisting outward thrust would be the toenails through the rafters into the top plate.
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Old 07-17-2014, 09:55 PM   #4
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Default Re: Technical Design Question, Wooden walls still need Tension band?

Quote:
Originally Posted by CrawDaddy View Post
Thanks for the reply,

Is this really the case? If you have your forces, as in weight of the roof, coming to the wall at an angle, doesn't there have to be some kind of horizontal component resisting the thrust? This is achieved by the tension band in yurts and most commonly in my experience with rafter ties on a gable roof for instance. This is the part I'm confused about, because I know that the top plate could be fashioned as one continuous piece of wood, but don't know how strong that would be. Also, unless I used brackets, hurricane clips or the like, the only connection resisting outward thrust would be the toenails through the rafters into the top plate.
Hummm I guess I was picturing something different with your roof, and the more I think about the roof and walls the more I believe you will need either a bracket to connect the frames like you said or a cable. A toe nail isn't going to hold the load coming down that rafter.
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Old 07-18-2014, 09:38 AM   #5
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Default Re: Technical Design Question, Wooden walls still need Tension band?

I think a twelve sided building, 12' in diameter, would look cool. It you have a twelve rafter roof, the building is definitely going to look better with twelve flat walls. I'd forget messing with circular plates. I'd use 2x4 studs on the wall ends. Forget using 4x4s.

You can easily production cut all 24 2x4 wall plates, (15 degree angle), nail up the wall sections, and quickly erect the walls. Install a stud in the middle of each 3' long wall section that doesn't have a window, or the door, for nailing up sheet goods.

Lap the double plate as you said. Tie the top plates together with 1 foot Simpson Strong Tie metal straps running horizontally at the outside plate joints. Anchor the rafters to the plates with Strong Tie rafter brackets, 1 on each side of each rafter, oriented on the inside of the building. 1x6 t&g roofing solidly nailed to the rafters will prevent the roof from expanding.

I'd like to suggest a roof overhang would be a good idea on this building. Run the rafters past the outside plate line by a half foot to a foot, depending on exterior wall height. Buildings look good with an overhanging roof.

Will be plenty strong and straightforward to build. Easy to finish off because all surfaces are planar. Will withstand the weather fine. Plus is gonna look REAL cool!

Have fun.
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